Removes ignore D4000 of linter.

[caduvieira]

What is the purpose of this Pull Request?
Removes 'Ignore D4000" Maintainer and Label
See hadolint/hadolint#93 (comment)

How to test if it really works?
docker run --rm -i lukasmartinelli/hadolint - < Dockerfile

Who can help reviewing it?
@anaschwendler

[cuducos]

1. Used GitHub instructions to pull the branch to my local machine:
   git checkout -b caduvieira-travis_fix master && git pull https://github.com/caduvieira/rosie.git travis_fix
2. Ran the suggested command (used in the CI):
   docker run --rm -i lukasmartinelli/hadolint - < Dockerfile
3. Got the following error:

container_linux.go:265: starting container process caused "exec: \"-\": executable file not found in $PATH"
docker: Error response from daemon: oci runtime error: container_linux.go:265: starting container process caused "exec: \"-\": executable file not found in $PATH".

Sounds like a weird Docker issue, not a problem with the PR. Any ideas on an workaround so I can test it? Tested with zsh and bash BTW…

[caduvieira]

Sorry i forgot the '-' in the command

[cuducos]

Great then ; ) Done!